The long-range goals of this research program are to understand the cellular consequences of Leber congenital amaurosis - 1 (LCA1) and to determine if therapies based on recombinant lentiviral vectors can be used to treat this devastating autosomal recessive retinal disease that results in blindness or severe visual loss in newborn infants. LCA1 is caused by mutations in the gene encoding photoreceptor guanylate cyclase -1 (GC1), the majority of which lead to loss of function of the GC1 enzyme. The GUCY1*B chicken, which carries a null mutation in GC1, is a naturally occurring model of LCA1 that possesses a cone-dominant retina and is blind at hatching. Analyses of this model, which serves as a focal point for this research program, provide a unique opportunity to improve our understanding of LCA1 and of cone responses to disease. Aim 1 of this proposal is to test the hypothesis that the expression of normal GC1 in GUCY1*B photoreceptors is sufficient to rescue photoreceptor function and prevent degeneration. Lentivirus carrying a GC1 transgene will be administered to GUCY1*B chickens during embryonic development and the effectiveness of the treatment will be assessed at 2-4 weeks of age by examining (1) the behavior of the animal, (2) the responsiveness of the retina to light by electroretinography, (3) retinal morphology, and (4) the expression of GC1 and guanylate cyclase activating protein -1. Aim 2 of this proposal is to determine the relationship between disease progression in GUCY1*B retina and the ability of our gene replacement strategy to rescue photoreceptor function. Since the GUCY1*B photoreceptor population is 80% cone, these studies will be particularly relevant to understanding cone disease and the responses of these cells to therapeutic treatment. Expression of the GC1 transgene in retina will be delayed until 10, 21 or 60 days after hatching and will be controlled by either placing the GC1 transgene under the control of a tetracycline-regulatable promoter or by varying the time of administration of the GC1 lentivirus. The efficacy of the delayed treatments will be assessed as described for Aim 1. The results of these experiments will improve our understanding of LCA1 and the potential usefulness of gene therapy to treat this disease. In addition, they will provide new information about the effectiveness of tetracycline-regulated expression systems to control the expression or therapeutic genes in the retina.